A current amplifier (current-to-voltage converter) is commonly configured using an operational amplifier (op-amp). FIG. 15 depicts a current amplifier.
In this current amplifier, the relationship between voltage output Vout, current input Iin and feedback resistance Rf in the frequency range where open loop gain of the op-amp is great is as the following formula (1).Math. 1Vout=−(Iin×Rf)  (1)The current amplification factor is determined by the feedback resistance Rf. For example, if the current of +1 nA flows as the current input Iin when the feedback resistance Rf is 1 GΩ, the voltage of −1 V is generated as the voltage output Vout. If the current of −1 nA flows as the current input Iin when the feedback resistance Rf is 1 GΩ, the voltage of +1V is generated as the voltage output Vout.
In this current amplifier, if inverting input capacitance Csin, which comes from capacitance of an input cable, capacitance between input terminals of the op-amp, stray capacitance, and so on, exists in the current input (inverting input of the op-amp=virtual ground point) Iin, the bandwidth fc of flat frequency response (that is, no peak, no mid range attenuation, no soft knee characteristic, etc.), which is the widest, can be obtained under the condition that feedback capacitance Cf is the following formula (2) (Equation 3 in Design Considerations for a Transimpedance Amplifier. See Citation List below).
      Math    .                  ⁢    2                                Cf          ≈                                                    C                ⁢                                                                  ⁢                sin                                            2                ⁢                                  π                  ·                  Rf                  ·                  ft                                                                                          (          2          )                    The bandwidth fc in this case is as the following formula (3) (Equation 4 in Design Considerations for a Transimpedance Amplifier).
      Math    .                  ⁢    3                                fc          ≈                                    ft                              2                ⁢                                  π                  ·                  C                                ⁢                                                                  ⁢                                  sin                  ·                  Rf                                                                                          (          3          )                    
Here, “ft” is a gain bandwidth product (unity-gain bandwidth) of the op-amp. The relationships of the above formulae are based on the premise that the open loop gain of the op-amp in the low frequency range is great enough (for example, at least 60 dB).
A common coaxial cable has the capacitance of about 100 pF per meter. If a current signal source and a current amplifier are connected via such a coaxial cable and if the connecting coaxial cable is about 10 m, the inverting input capacitance Csin is about 1,000 pF.
The art expressed in JP 2005-064903 A is used for achieving a broadband current amplifier etc. The amplifier expressed in JP 2005-064903 A uses a current feedback op-amp. Also, a buffer of high input impedance is provided before inverting input, and an integrator is provided before non-inverting input.
Datasheet LCA-4K-1G (see Citation List below) expresses the datasheet of a current amplifier, which is the high amplification factor (current amplification factor: 109 V/A=1 GΩ) and low noise.